Chemical activation in unimolecular reactions of some unsubstituted alkoxy radicals

Abstract

In a previous work, (F. Caralp, W. Forst and M. T. Rayez, Phys Chem. Chem. Phys., 2003, 5, 476.), we presented a statistical RRKM-master equation (RRKM-ME) procedure for the calculation of the fraction of chemically activated β-hydroxy-alkoxy radicals, produced by the reaction RO₂ + NO → RO + NO₂, which undergo “prompt” unimolecular reactions (on a sub-nanosecond time-scale). In experimental relative-rate studies of thermal unimolecular reactions of several unsubstituted alkoxy radicals produced by this same reaction (H. G. Libuda, O. Shestakov, J. Theloke and F. Zabel , Phys. Chem. Chem. Phys, 2002, 4, 2579, and Geiger et al., J. Atmos. Chem. 2002, 42, 323), a systematic increase of the effective rate constant ratio (k_diss/k_O2)_eff with a partial pressure of O₂ was observed. This O₂ dependence was attributed to the formation and decomposition of chemically activated alkoxy radicals. In this work, it is shown that the same statistical RRKM-ME procedure quantitatively predicts the chemical activation effect observed experimentally. The good agreement between experiment and calculation confirms the interpretation of the experimental results.